Bacterial cells have been genetically engineered to produce useful…

The idea

Genetic engineering transfers a desired gene into a bacterial cell so the bacterium makes a human protein. For insulin, the human insulin gene is cut out using enzymes and inserted into a plasmid vector, which is put into a bacterium. The bacterium is grown in a fermenter, where it multiplies and expresses the gene, producing human insulin that is extracted and purified. This works because bacteria reproduce rapidly and can be grown on a large scale. The insulin is identical to human insulin, so it is less likely to cause an immune reaction than animal insulin, and it can be made in unlimited quantities. This links gene transfer, protein synthesis, bacterial growth and medical use.

What earns the marks

• The human insulin gene is removed from a human chromosome using enzymes that cut DNA at specific base sequences.
• The gene is inserted into a vector, usually a bacterial plasmid, using enzymes that join DNA fragments together.
• The recombinant plasmid is transferred into a bacterial cell, which then multiplies by binary fission.
• The genetically engineered bacteria are grown on a large scale in a fermenter under controlled conditions.
• The bacteria transcribe and translate the human gene, producing human insulin protein.
• The insulin is extracted and purified before being used to treat people with diabetes.

Common mistakes and how to put them right

• Thinking the bacterium makes insulin because it naturally contains the human gene.

  Put it right: The human gene must first be cut out and inserted into the bacterium.

• Confusing the roles of the cutting and joining enzymes.

  Put it right: One type of enzyme cuts the gene and plasmid, and another joins the gene into the plasmid.

• Believing bacteria can produce any human protein without a vector.

  Put it right: The gene must be carried into the bacterium, usually by a plasmid.

Exam tips

• Name the vector and explain why a plasmid is suitable, for example it is small and can be taken up by bacteria.
• Sequence the stages clearly: cut out gene, insert into vector, transfer into bacterium, grow, express, extract and purify.
• Link the medical benefit to the product being human insulin, which reduces the risk of rejection compared with animal insulin.